Source Based Version Pinning: String/Simple Form doesn't work

### Describe the Bug

If you copy this the example (from https://atmos.tools/design-patterns/version-management/source-based-versioning#string-form-simple):

```yaml
components:
terraform:
vpc:
source: "github.com/cloudposse/terraform-aws-components//modules/vpc?ref=1.450.0"
vars:
cidr_block: "10.0.0.0/16"
```

And try something like `atmos list stacks` you get an error message:
> failed to load component config: invalid component source section: 'components.terraform.vpc.source' in the file '...'

Which makes sense since the code at https://github.com/cloudposse/atmos/blob/v1.214.0/internal/exec/stack_processor_utils.go#L1900 checks for a `map[string]any`.

The Map Form works.

### Expected Behavior

It should work as documented (or the documentation amended).

### Steps to Reproduce

See above.

### Screenshots

_No response_

### Environment

- Version: 1.214.0

### Additional Context

_No response_